1. Field of the Invention
The present invention relates generally to the field of fiber optic networks and more specifically to a tunable, single-channel dispersion compensator for high-speed optical systems.
2. Description of the Related Art
Fiber optic communication systems use wavelength division multiplexing (WDM) to transfer large amounts of data at high speeds. In order to use WDM, channels are specified within a wavelength band. For example, it is generally accepted that the C-band begins at 1530 nm and extends to 1565 nm, and each channel in the C-band is approximately 0.8 nm wide and corresponds to a channel spacing of 100 GHz. The International Telecommunications Union (ITU) sets the standards for channel spacing, channel width and other communication band parameters for this and other optical communication bands.
Lasers that transmit data on optical media, such as optical fibers, provide a narrow spectrum of light (i.e., a light pulse) that includes many wavelengths. Chromatic dispersion is a variation in the velocity of this light according to wavelength. Among other things, this variation in velocity causes the light pulses of an optical signal to broaden as they travel through the optical media. This phenomenon, known as “pulse spreading,” can cause increased bit error rates if the light pulses spread to a point where they begin to overlap with one another.
Chromatic dispersion is particularly problematic in high-speed optical systems because the light pulses associated with higher bandwidths have broader wavelength spectra, resulting in relatively more pulse spreading, and the light pulses typically are narrower and transmitted closer together. The combination of these factors creates a system more susceptible to light pulse overlap and increased bit error rates.
As is also known, chromatic dispersion is the rate of change of the group delay response of the light pulses of an optical signal as a function of wavelength. Thus, one approach to compensating for chromatic dispersion involves passing the optical signal through a dispersion compensator that exhibits a rate of change of the group delay response as a function of wavelength opposite to that caused by the optical medium.
For example, U.S. Pat. No. 6,724,482 presents a dispersion compensator that includes a series of cascaded Gires-Tournois interferometers (i.e., GT etalons). Each GT etalon in the dispersion compensator has an individual group delay response. The group delay response of the dispersion compensator (hereinafter referred to as the “aggregate group delay response”) is the summation of the individual group delay responses of each of the cascaded GT etalons. The disclosed dispersion compensator is designed such that the aggregate group delay response across a channel has a rate of change as a function of wavelength opposite to that caused the optical medium, thereby compensating for the chromatic dispersion within a single channel of a multi-channel WDM communication system.
In addition, a GT etalon has a periodic group delay response that repeats as a function of wavelength. The free-spectral-range (FSR) is a device parameter of a GT etalon that determines, among other things, the period of the group delay response. Based on these principles, U.S. Pat. No. 6,724,482 also teaches that by designing each GT etalon to have an FSR that aligns with the ITU's channel spacing scheme, the disclosed dispersion compensator can provide dispersion compensation across several channels simultaneously. For example, in a multi-channel WDM communication system having a 100 GHz channel spacing, if the dispersion compensator includes only GT etalons having an FSR of 100 GHz, then the dispersion compensator will provide the same aggregate group delay response, and therefore dispersion compensation across each channel of the system.
One drawback of this type of dispersion compensator is that the passband is limited by the relatively high insertion losses associated with each GT etalon stage. More specifically, in order to extend the passband of the dispersion compensator, more GT etalons must be used. However, the additional GT etalons increase the total insertion losses across the dispersion compensator, which is undesirable. Thus, to keep the total insertion losses at an acceptable level, only a limited number of GT etalons can be used in the dispersion compensator. Limiting the number of GT etalons, however, results in a passband that is not optimized for high-speed optical systems, such as 40 Gb/s per channel optical systems.
As the foregoing illustrates, what is needed in the art is a dispersion compensator with an increased passband for high-speed optical system applications that does not introduce increased insertion losses.